The present invention relates, in general, chemical sensing and, more particularly, to an improved chemical sensor providing both selectivity and high sensitivity.
Chemical sensing is fundamental to economic development, national security, and the quality of life. The demand for better sensing or detection technologies is ever-increasing to address needs in many different areas, such as the detection of concealed explosives in airports, chemical warfare agents that are fatal at extreme trace levels, or chlorine produced by chemical plants. To be effective, a chemical sensing technology must provide sufficient sensitivity and selectivity. Stability, robustness, and portability are also necessary or at least highly desirable characteristics. Therefore, any significant advance in current chemical sensing technology that improves sensitivity, selectivity, or adaptability will have a significant impact on national and global needs.
Although many transduction mechanisms exist for chemical sensing, optical absorption, in particular, is widely used. The ultimate sensitivity of an optical absorption measurement is limited by quantum noise arising from the discrete nature of light, although this limit is rarely achieved in practice. Recently, with the development of cavity ring-down spectroscopy (CRDS), the potential for routine quantum noise limited optical absorption measurements has become apparent. (See R. D. van Zee, J. T. Hodges, and J. P. Looney, App. Opt. 38, 3951 (1999)).
The principles and applications of CRDS are discussed, e.g., in (See A. O""Keefe and D. A. G. Deacon, Rev. Sci. Instrum. 59, 2544 (1988); Cavity-Ringdown Spectroscopy, K. W. Busch and M. A. Busch, eds. Coxford University Press, 1999) and these references, among others, may be consulted for a more complete discussion of CRDS. However, in brief, a typical gas-phase CRDS experiment, a stable optical cavity is formed from a pair of concave, highly reflective mirrors. When light, usually from a pulsed laser source, is injected into the cavity, the intensity of the circulating light decays exponentially with a frequency-dependent xe2x80x9cring-downxe2x80x9d time, xcfx84(xcfx89), given by the ratio of the round-trip time, tr, to the sum of the round-trip losses, or       τ    ⁡          (      ω      )        =            t      r                                L          o                ⁡                  (          ω          )                    +                        L                      a            ⁢                          xe2x80x83                        ⁢            bs                          ⁡                  (          ω          )                    
where L0 (xcfx89) is the intrinsic cavity loss and Labs(xcfx89) arises from absorption by gases contained within the cavity. The difference in intensity decay rates for gas-filled and empty cavities, as a function of laser frequency, provides the absolute absorption spectrum of the sample. Since the intensity decay rate (xcex11/xcfx84) is employed instead of a ratio of intensities, as in conventional absorption spectroscopy, the measurement is essentially immune to noise introduced by light source intensity fluctuations. The minimum detectable absorption in CRDS can be expressed as the product of the relative uncertainty in the ring-down time and the intrinsic cavity loss, or (Labs)min=L0*(xcex94T/T)=L0*2"sgr"T/(TN) where "sgr"T is the standard deviation of the ring-down time and N is the number of decay times averaged. (See P. Zalicki and R. N. Zare, J. Chem. Phys. 102, 2708, (1995); D. Romanini and K. K. Lehmann, J. Chem. Phys. 99, 6287-6301, (1993).) This expression for (Labs)min reveals both the simplicity and challenge of CRDS: minimize the intrinsic cavity loss and determine the ring-down time with the highest possible precision.
A variant of CRDS, termed evanescent wave cavity ring-down spectroscopy (EW-CRDS), has recently been developed, which permits application of CRDS to surfaces, films, and liquids. (See A. C. R. Pipino, J. W. Hudgens, R. E. Huie, Rev. Sci. Instrum. 68 (8), 2978, (1997); A. C. R. Pipino, J. W. Hudgens, R. E. Huie, Chem. Phys. Lett. 280, 104 (1997); A. C. R. Pipino in Proceedings of SPIE, Vol. 3535, Boston, Mass. (1998); A. C. R. Pipino, Phys. Rev. Lett. 83 (15), 3093-3096, (1999); A. C. R. Pipino in Proceedings of SPIE, 3858, Boston, Mass., (1999); A. C. R. Pipino, Appl. Opt. 39 (9), 1449 (2000); U.S. Pat. Nos. 5,835,231; 5,835,231; 5,986,768.) This technology is described in some detail in these references but in brief, EW-CRDS employs intracavity total internal reflection (TIR) to generate an evanescent wave at a resonator surface that allows optical absorption of condensed matter to be probed in a manner similar to attenuated total reflection (ATR) spectroscopy (see N. J. Harrick, Internal Reflection Spectroscopy, (Interscience Publishers, New York, (1967)), but with much higher sensitivity. In particular, a minuscule fraction ( less than 10xe2x88x924) of a molecular layer of molecules can be detected at the TIR surface with EW-CRDS. Several resonator designs have been demonstrated for EW-CRDS, including variations that permit a miniature, robust optical absorption sensor to be achieved, thereby facilitating portability.
In many chemical sensing applications, detection of the analyte at a surface by direct absorption has major advantages. However, the analyte must have a significant absorption cross-section (or molar absorptivity) at the probe wavelength, which limits the minimum analyte concentration that can be detected. Typically, absorption cross-sections are largest for electronic transitions occurring in the visible region of the spectrum. Operation in the visible region also benefits from the availability of inexpensive sources including diode lasers, low-noise high-quantum efficiency detectors, and high transmission optical materials. However, many chemical species of interest do not have a significant visible absorption, and show instead significant absorption in the ultraviolet or infrared spectral regions. As discussed below, one aspect of the invention concerns circumventing this limitation.
One chemical sensing strategy that employs visible absorption, but permits detection of analytes that do not absorb at the probe wavelength, involves the use of surface plasmon polariton resonance (SPPR). This technology is described, for example, in J. Homola, S. S. Yee and G. Gauglitz, Sens. Act. B, 54, 3, (1999). In brief, SPPR is a surface electromagnetic wave that arises from the collective excitation of free electrons. A typical, conventional apparatus for making a SPPR measurement is shown at 10 in FIG. 1. In apparatus 10, a metal film 11 is deposited on the base of a prism 12, forming a three layer system consisting of the prism 12, the metal film 11, and the ambient medium indicated at 13. A visible laser beam, or a light beam from another visible light source, is denoted 14 and is incident on the metal film 11 at an angle of incidence xcex8i that exceeds the critical angle, defined by xcex8c=sinxe2x88x921(no/ni) where ni and no are the refractive indexes of the material of the prism 12 and the ambient medium 13, respectively. Since xcex8i greater than xcex8c, total internal reflection occurs, giving rise to an evanescent wave 15. (See also N. J. Harrick, Internal Reflection Spectroscopy, (Interscience Publishers, New York, (1967).) For a certain angle xcex8r, with xcex8i=xcex8r greater than xcex8c, the evanescent wave 15 generated at the prism-metal interface excites the SPPR at the metal/ambient medium interface. The SPPR efficiently absorbs the incident light, trapping the electromagnetic energy in the form of a surface wave with a locally enhanced electric field.
The SPPR apparatus 10 is highly sensitive to environmental conditions at the metal/ambient medium interface. Hence, the angle of resonance, xcex8r, or the absorbance magnitude at a given xcex8i near xcex8r, are very sensitive to chemical and physical interactions at the interface. In some cases, a reaction of the analyte occurs directly with the metal of the metal film 11. In other cases a thin film is applied to the metal that responds selectively to the analyte, changing in the local environment sensed by the SPPR apparatus 10. Both of these types of interactions can be highly selective for the specific analyte of interest. Sensors based on SPPR have been successful in both research and commercial applications. However, significant improvements in sensitivity are needed.
In accordance with the invention, a chemical sensor is provided which comprises an optical resonator with a nanostructured surface or surfaces that permit highly sensitive and selective chemical detection by absorption spectroscopy, advantageously in the visible spectral region. An important advantage of the invention is that the analyte is not required to have significant absorption cross section at the probe wavelength because, in contrast to conventional spectroscopy, detection is of the absorption of one or more of the nanoparticles bound to the resonator surface and forming the nanostructured surface. The nanoparticles have an enormous absorption cross section which is highly sensitive to the dielectric properties of the particle or the environment thereof and this enables the highly sensitive chemical detection mentioned above.
Generally speaking, the present invention relates to a chemical sensor comprising an optical resonator including a nanostructured surface comprising a plurality of nanoparticles bound to at least one surface of the resonator.
Preferably, the nanoparticles provide optical absorption and the sensor further comprises means for detecting the optical absorption of at least one of said nanoparticles.
In a preferred implementation, a selective chemical interaction is provided which modifies the optical absorption of one of (i) the at least one nanoparticle and (ii) the environment of the at least one nanoparticle, and an analyte is detected based on the modified optical absorption.
Advantageously, the sensor further comprises means for generating a light pulse which enters the resonator to interrogate the modified optical absorption, the detecting means comprising a detector for detecting the light pulse when the light pulse exits the resonator.
In one preferred embodiment, the selective chemical interaction mentioned above is provided by a direct chemical interaction between the at least one nanoparticle and the analyte which alters the absorption of the at least one nanoparticle.
In an alternative preferred embodiment, the at least one nanoparticle comprises a coated nanoparticle having a coating that selectively binds to the analyte to produce an effective coating refractive index change and the aforementioned selective chemical reaction comprises the selective binding of the coating to the analyte.
Advantageously, the at least one nanoparticle comprises a plurality of nanoparticles which support a surface plasmon polariton resonance.
In an advantageous implementation, the at least one nanoparticle comprises a nanoparticle selected from the group consisting of gold, silver, cadmium sulfide and zinc selenide nanoparticles.
In a further advantageous implementation, the at least one nanoparticle comprises a nanoparticle selected from the group consisting of spherical, spheroidal, and tetrahedral nanoparticles. In a particularly beneficial embodiment for some applications, the at least one nanoparticle comprises a gold nanosphere.
In one preferred embodiment, the at least one surface comprises an ultra-smooth polished surface and the optical resonator comprises a resonator employing intracavity total internal reflection so as to permit the use of evanescent wave cavity ring-down spectroscopy in probing the modified optical absorption.
In one important application, the sensor is used to detect NO2 and nitrocompounds and the nanoparticles comprise gold nanoparticles.
In a further important application, the sensor is used to detect volatile organic compounds and said nanoparticles have a coating of cyclodextrin molecules.
In accordance with a further aspect of the invention, a chemical sensor is provided which comprises a resonator providing intracavity total internal reflection and comprising first and second opposed planar coated facets and a further convex facet acting as a total internal reflection surface; a light source for producing a light pulse which enters through said first coated surface and exits through said second coated surface; a plurality of nanoparticles covalently attached to said convex surface so as to absorb an evanescent field produced by said convex surface.
Preferably, the nanoparticles comprise gold nanospheres. Advantageously, the light pulse comprises a laser pulse.
According to a further aspect of the invention, there is provided a chemical sensor comprising a resonator defining a cavity, a light source for generating light which enters said cavity, the resonator providing intracavity total internal reflection of said light, and including at least one surface having a plurality of nanoparticles bound thereto such that the optical absorption of at least one of the nanoparticles, or of the environment of the nanoparticles, is modified in response to a selective chemical interaction, the sensor further comprising means for detecting an analyte based on the modified optical absorption.
As above, in one implementation, the selective chemical interaction is provided by a direct chemical interaction between the at least one nanoparticle and the analyte which alters the absorption of the at least one nanoparticle.
Also as above, in an alternative implementation, the at least one nanoparticle comprises a coated nanoparticle having a coating that selectively binds to the analyte to produce an effective coating refractive index change and said selective chemical reaction comprises the selective binding of the coating to the analyte.
Again, the at least one nanoparticle preferably comprises a nanoparticle selected from the group consisting of gold, silver, cadmium sulfide and zinc selenide nanoparticles, and in some important applications, comprises a plurality of gold nanospheres.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.